Nickel-chromium alloys



June 23, 1970 D. M. WARD ETAL 3,516,826

NICKEL-CHROMIUM ALLOYS Filed Aug. 14. 1968 Z (/Yb +0.5 7a) United States Patent 3,516,826 NICKEL-CHROMIUM ALLOYS David Marshall Ward, Birmingham, Paul Isidore Fontaine, Solihnll, and Michael John Fleetwood, Berkhampsted, England, assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware Filed Aug. 14, 1968, Ser. No. 752,549 Claims priority, application Great Britain, Aug. 18, 1967, 38,280/ 67 Int. Cl. C22c 19/04 US. Cl. 75171 13 Claims ABSTRACT OF THE DISCLOSURE Nickel-chromium-iron alloys having high strength at elevated temperatures combined with corrosion-resistance and structural stability contain from 30.8 to 60% nickel together with controlled amounts of titanuim, niobium and/or tantalum, and optionally also one or more of tungsten, molybdenum, aluminum, boron, zirconium, rare earth metal and yttrium in controlled amounts. The alloys are useful for boiler superheater tubes.

The superheater tubes of steam boilers operating at temperatures up to 650 C. and above are subjected to high stresses for very long periods at the operating temperature and at the same time to very severe corrosive attack on the fire side. They must therefore be made of alloys that have high stress-rupture strength; do not undergo structural changes leading to embrittlement and loss of impact strength on prolonged heating during periods of many years; and have adequate resistance to corrosive attack at the operating temperature by the combustion products of the fuel. This last requirement is particularly severe, since the coals used to fire large industrial boilers, e.g. for electricity generating plant, generally contain sulphur and chlorides and the resulting fuel ashes are extremely corrosive.

It is well known that the resistance to corrosion of nickel-chromium-iron alloys increases with increasing chromium content. At the same time, however, there is a substantial increase in their susceptibilty to embrittlement on prolonged heating. For this reason it has not been possible to achieve the desired strength in alloys of high chromium content while maintaining adequate freedom from emrbittlement, and the nickel-chromium-iron alloys hitherto used for superheater tubes have generally not contained more than 18% chromium.

It is an object of this invention to provide nickelchromium-iron alloys having an improved combination of high-tempeatture strength, structural stability and resistance to corrosion.

A further object is to provide wrought articles and parts for use under stress at elevated temperatures, eg boiler superheater tubes, having a high level of structural stability and resistance to corrosion by fuel ash and the gaseous combustion products of sulphur-containing fuel.

Other objects and advantages of the invention will appear from the following description and the accompanying drawing, in which contents of titanium, in percent by weight, are plotted as abscissae against total contents of niobium and tantalum expressed as the sum (Nb+0.5 Ta) in weight percent as ordinates.

3,516,826 Patented June 23, 1970 The present invention is based on the surprising discovery that the corrosion resistance of nickel-chromiumiron alloys having chromium contents substantially above 18% is greatest with nickel contents of about 30 up to 60%, and that, by employing titanum, niobium and tantalum in controlled amounts as hardening elements in these most corrosion-resistant alloys, high stress-rupture strength together with good corrison resistance and adequate stability can be achieved.

According to the invention nickel-chromium-iron alloys contain (in weight percent) from 32.8 to 60% nickel, from 24 to 31% chromium, from 0.01 to 0.15% carbon, at least one of titanum, niobium and tantalum in an amount corresponding to a point in the area ABCDEA in the accompanying drawing, in which the abscissae represent titanium contents and the ordinates represent the sum of the niobium and half the tantalum content.

The alloys preferably contain titanium, since in its absence they are difiicult to work, and for the highest stressrupture strength at least 0.5% titanium is present. When the sum of the niobium and half the tantalum contents does not exceed 1.6% higher titanum contents are advantageous, the lower limit being given by the line BF in the drawing.

Of the elements niobium and tantalum, we prefer to employ niobium, and most advantageously the alloys contain both titanium and niobium. As is well known, however, commercial niobium usually contains some tantalum, and if desired tantalum may deliberately be added to replace part or all of the niobium. Whether or not niobium or tantalum is present, the proportions of titanium, niobium and tantalum must be controlled so as to lie in the area ABCDEA, and preferably in the area ABFDEA, in order to achieve adequate stress-rupture strength and freedom from embrittlement.

The line ABF in the drawing may be represented by the equation:

(percent Nb) +0.5 (percent Ta) +1.7(percent Ti) 1.7

and the line BC by the equation:

0.18 (percent Nb) +0.9 (percent Ta) (percent Ti) =0.342

Alloys with chromium contents less than 24% have relatively poor corrosion resistance and at chromium contents greater than 31% the alloys are insufficiently workable. Preferably the chromium content does not exceed 27%.

The alloys may contain other elements, that is to say from 0 to 3% tungsten, 0 to 3% molybdenum, 0 to 0.5% aluminium, 0 to 0.01% boron, 0 to 0.1% zirconium, 0 to 0.3% of rare earth metal and 0 to 2% yttrium. While to ensure maximum corrosion resistance the content of nickel must be between 32.8 and 60%, to ensure that adequate freedom from embrittlement is also maintained the content of nickel must also be correlated with the contents of other elements present in the alloy. For chromium contents not exceeding 27%, the nickel content must comply with the relationship:

percent Ni 2 (percent Cr) +4(percent Nb) -|-2(percent Ta)+8 (percent Ti-l-percent A1) +0.6(percent W) +1.2(percent Mo) 22 If the chromium content exceeds 27%, the minimum nickel content must be increased above the minimum 3 given by the relationship just set forth by 1% for each additional 1% of chromium present, so that percent Ni) 3 (percent Cr) +4 (percent Nb) 2 (percent Ta) +8 (percent Ti+percent Al) +0.6 (percent W) +1.2(percent M) 49 The nickel content is preferably from 42 to 50% The balance of the alloy, except for impurities, is iron. At least 10% iron must be present for adequate corrosionresistance. Of the elements commonly present as impurities, silicon has a markedly deleterious effect on the impact strength and the suscepitibility of the alloys to embrittlement. Its content must therefore be kept below 1% and preferably below 0.5%. Manganese may be present as an impurity in amounts up to 1%, but preferably the manganese content does not exceed 0.5%.

At least 0.01% of carbon is required for adequate stressrupture ductility, and generally the alloys will contain at least 0.02% carbon, but more than 0.15% leads to reduced creep resistance and leads to difficulties in hot working. Preferably the carbon content does not exceed 0.06%. Boron and zirconium contribute to stress-rupture strength and ductility, and should be present in small amounts if the highest level of these properties is required. However, these elements impair the weldability of the alloys, and if the alloy is to be welded under particularly severe conditions of restraint, boron and zirconium should be absent.

The resistance of the alloys to oxidation and scaling is improved by the presence of rare earth metals, and one or more of these metals may be added, for example in the form of Mischmetall. Advantageously from 0.01 to 0.3% of rare earth metal, for example from 0.03 to 0.08%, is added. We find that yttrium additions also improve the oxidation and scaling resistance of the alloys and their resistance to sulphidation, and yttrium may advantageously be added in amounts from 0.2 to 2%, for example from 0.5 to 1%. A particularly advantageous range of compositions for the alloys is as follows:

Nickel 42 to 50%, chromium 24 to 27%, carbon 0.02 to 0.15%, titanium 0.5 to 2%, niobium 0 to 4%, the contents of titanium, niobium and any tantalum present as an impurity corresponding to a point in the area ABFDEA in the drawing, iron and impurities, balance.

The alloys are particularly suitable for use in the wrought form, and may be hot-worked by conventional methods, e.g. forging, hot-rolling and extrusion, to the usual mill forms, including plate, bar and sheet. They may also be drawn to wire. After working they are advantageously given a stress-relieving heat-treatment in the temperature range 900 to 1250 C. The alloys with the highest contents of hardening elements within the ranges set forth may be age-hardened. For those alloys heating at 900 to 1250 C. also serves as a solution treatment, which may be followed by aging in the range 600 to 850 C., e.g. for from 4 to 24 hours.

In order to give those skilled in the art a better understanding of the invention, the following illustrative examples are given:

The compositions of eight alloys according to the invention (Alloys 1 to 8) and of five comparative alloys (Alloys A to E), together with the results of test on them are given in Table 1. In the tests, specimens of each alloy were immersed in a salt mixture consisting of 35.6% Na S0 35.6% K 80 23.8% Fe O and 5% NaCl (per centages by weight) and heated to 650 C. in a furnace through which a gas mixture composed of 81.15% N CO 3.6% 0 and 0.25% S0 (percentages by volume) was passed at a rate of 15 litres per hour. The corrosion damage was assessed by removing the corrosion products by cathodic descaling in molten sodium hydroxide and comparing the weight of each specimen with the initial weight before exposure in the corrosion test. The salt mixture in which the specimens were im- 4 mersed is similar in composition to fuel ash, so that the specimens were subjected to a very concentrated and aggressive mixture, in a simulated coal-fired combustion atmosphere. A test of 500 hours in such conditions is not dissimilar to that which would be experienced by the specimen in many months in practice. The most resistant materials are those which show the least loss in weight. Table 1 also includes Alloy T, which is an austenitic stainless steel currently used as the material of superheater tubes and which contains 0.25 vanadium in addition to the elements shown.

It will be seen that Alloys A, B and C contained less than the minimum amount of chromium required by the invention, and Alloys C and D had nickel contents greater than the maximum permitted value. It is clear from Table 1 that Alloys 1 to 8 show markedly greater resistance to corrosion than the remaining alloys.

The necessity of combining the hardening elements in the correct proportions in order to achieve good stressrupture strength is illustrated in Table 2, which includes other alloys (Alloys 9 to 15) in accordance with the invention, and six additional comparative alloys (F, G, H, I, K and L), which in common with Alloy A do not contain enough niobium and titanium. It will be seen that Alloys 1 to 5 and 8 to 15 had stresses for rupture in hours at 650 C. not less than 18 ton f./in. whereas Alloys A, F, G, H, J, K and L had inferior rupture strengths, the stresses for rupture in 90 hours at 650 C. being not more than 17 ton f./in.

Measurements of impact strength after prolonged heating are set forth in Table 3. The alloys were tested after heating at 650 C. for times of 1,000 and 10,000 hours and also after 25% cold-working followed by heating for 1,000 hours at 650 C. This last treatment accelerates any tendency a material may have to embrittle in longtime exposure, and is equivalent to heating at 650 C. for times considerably in excess of 10,000 hours. The table shows seven further comparative Alloys M, O, P, Q, R, S and U, in none of which are contents of nickel, chromium, titanium, niobium and tungsten correlated as required by the invention. The alloys were subjected to three different heat treatments, as follows:

I-heated for 1,000 hours at 650 C.;

IIheated for 10,000 hours at 650 C.;

III-cold-worked 25 and heat-treated for 1,000 hours at 650 C.

It will be seen that Alloys 1 to 5, and 7, 8, 14 and 15 retained an impact strength in excess of 25 ft. lb. after heating for 1,000 hours and not less than 15 ft. lb. even after cold-working and heating for 1,000 hours, whereas in the other alloys the impact strengths after cold-Working and heating for 1,000 hours were below 13 ft. lb.

The combination of high-temperature strength, corrosion-resistance and structural stability of the alloys according to the invention makes them particularly suitable for superheater tubes and other articles and parts that are exposed in use for prolonged periods at elevated temperatures, e.g. of 650 C. or above, to stress in the presence of fuel ash and gaseous combustion products of sulphur-containing fuelsjExamples of such articles and parts are parts of petrochemical installations, heatexchangers such as recuperators in steel plants, and furnace parts.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be re sorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

TABLE 1 Weight loss, Composition (weight percent) rug/em.

100 hrs. 500 hrs Ni Nb Ti .Al Mn B Zr exposure exposure 41. 8 1. 0.8 0.1 0.2 43 41. 9 2. 4 0. 35 0. 1 0.2 64 41.4 l. 26 0. 1 0.2 44. 2 1. 65 0.71 0. 1 0.2 11 44. 9 1. 95 0. 47 0. 1 0.2 31 49. 0 5. 2. 05 0.88 0. 1 0. 2 30 50.3 1. 45 0. 53 0. 1 0.2 28 54. 3 30. 1. 40 0. 62 0. 1 0. 2 16 41. 4 15. 1 1. 1 2. l 1. 2 0.48 0. 1 0.1 167 38. 1 18. 0 1. 65 1. 01 0. 60 0. 58 0. 27 128 1 68. 65 15. 0 3.0 2. 9 3. 3 0. 1 0. 58 0. 1 195 1 66. 64 30. 8 1. 55 0.93 0. 1 644 21. 62 24.42 0. 01 0. 1 0.03 0.92 156 11. 4 14. 6 1. l l. 1 0. 1 0. 1 5. 35 193 TABLE 2 Composition (weight percent) Stress for rupture in 90 hrs. at 650 C. 0 Ni Cr Nb Ti Al Mn Additional elements (ton f./in.

1 Calculated balance.

All samples heat-treated 1 hour 1,150 C. and air-cooled to room temperature.

TABLE 3 Composition (weight percent) Room temperature eharpy V-notch impact strength ft. lb. f.

Additional Fe 0 Ni Cr Nb T1 Al Mn elements I II III 1 Calculated balance.

W l i Mo)49, up to 0.01% boron, up to 0.1% 211"- 1. A nickel-chromium-lron alloy conslstmg of about comum, up to 0.3% rare earth metal, up to 32.8% to about 60% nickel, about 24% to about 31% chromium, about 0.01% to about 0.15% carbon, at least one metal selected from the group consisting of titanium, niobium and tantalum, the titanium being up to 2%, the niobium up to 4% and the tantalum up to 8%, with the proviso that the percentages of any titanium, niobium and tantalum being so correlated as to represent a po1nt in the area ABCDEA in the accompanying drawing, up to 3% tungsten, up toless than 2% molybdenum, up to 0.5% aluminum with the proviso that when the chromium content does not exceed 27% the nickel is present in an amount at least equal to the relationship 2 (percent Cr) +4(percent Nb) +[2(Tla)]2(percent Ta)+8(percent Ti+percent Al)+0.6(percent W)+1.2(percent'Mo)-22 and when the chromium content exceeds 27% the nickel is present in an amount at least equal to the relationship 3(percent Cr)+4(percent Nb)+[12(ITa)]2(percent Ta)+ 8(percent Ti+percent A1)+0.6(percent W)+1.2(percent 2% yttrium and the balance essentially iron, the iron constituting at least 10% of the alloy, said alloy having the properties of (a) a stress-rupture strength of at least hours at a temperature of 650 C. under a stress of at least 18 ton f./in. (b) an impact strength of at least 25 foot-pounds upon exposure to a temperature of at least 650 C. for a period of 1,000 hours, and (c) good resistance at a temperature of 650 C. to corrosive attack by the combustion products of sulfur-containing fuels.

2. An alloy in accordance with claim 1 containing about 24% to 27% chromium.

3. An alloy in accordance With claim 2 which contains at least 0.02% carbon.

4. An alloy in accordance with claim 3 in which the titanium, niobium and tantalum are correlated so as to represent a point in the area ABFDEA in the accompanying drawing.

5. An alloy in accordance with claim 1 in which the nickel content is from 42% to 50%.

6. An alloy in accordance with claim 4 in which the nickel content is from 42% to 50%.

7. An alloy in accordance with claim 1 in which the carbon content does not exceed 0.06%.

8. An alloy in accordance with claim 4 in which the carbon content does not exceed 0.06%.

9. An alloy in accordance with claim 1 in which titanium is present in an amount of at least 0.5%.

10. An alloy in accordance with claim 1 in which titanium is present in an amount of at least 0.5%.

11. An alloy in accordance with claim 1 Which contains 42% to 50% nickel, 24% to 27% chromium, 0.02% to 0.15% carbon, 0.5% to 2% titanium, up to 4% niobium, the titanium and niobium being correlated so as to represent a point in the area ABFDEA in the accompanying drawing.

12. An article or part formed from the alloy set forth in claim 1 and subjected in use for prolonged periods to 8 stress at elevated temperatures in the presence of the combustion products of sulfur containing fuels.-

13. An article or part formed from the alloy set forth in claim 4 and subjected in use for prolongedperiods to stress at elevated temperatures in the presence of-the combustion products of sulfur containing fuels.

References Cited UNITED STATES PATENTS 2,570,193 10/1951 Bieb er et al. 75-171 2,777,766 1/1957 Binder 75171 2,813,788 11/1957 Skinner 75'-17l 2,994,605 8/1961 Gill et al 75--171 RICHARD O. DEAN, Primary Examiner U.S. C1; X.R. 75-128 "14050 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5 Dated June 23, 1970 DAVID MARSHALL WARD, PAUL ISIDORE FONTAINE &: MICHAEL JOHN FLEETWOOI It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line #2, "0.9" should read -0.09--.

Columns 5 8c 6, Table I, second column heading "Fe" should read --Fe same Table, Alloy T next to last column, "7,198" should read --7l, 98-.

Columns 5 8c 6, Table III, under column heading Ni, Alloy l "41.9'

should read -44.9-, and Alloy 5 "M12" should read 44.2".

Column 5, line 70, delete [2(TTa) 1". Column 5, line 7M, delete "[12(T're)].

Column 7, line 1]. (Claim 10, line 1) "claim 1" should read --claim 4---.

SIGNED AND SEALED NBV 17m E R M Mum! mm 2. ill- Amng Offi Oomissiom at his. 

